This invention relates to a calibrated current source.
It is imperative that current sources, when used in certain arrays, maintain a stable, fixed current output relative to one another. For example, in digital to analog converters (DAC""s) a plurality of unit cells receive digital data that is converted to an analog output. For this purpose each unit cell includes a current source which to the extent possible is made identical with that of all of the other cell but still there are variations in the current source current outputs from cell to cell which introduces errors in the analog output. One approach to this problem employs a calibration technique in which all of the current sources are trimmed by increasing the current output by each of them to that of the maximum current output by any of them. Or choosing an intermediate value and increasing or decreasing the current provided by each to a defined level. This is done by switching each current source from its load, in a DAC the current switching circuit, to a calibration circuit which determines the value of current to be added or subtracted to meet the chosen level. While this has been successfully used, a further problem is introduced: when the switching between the load and calibration occurs, the voltage across the current source changes and since the current output varies as a function of the voltage across the current source, the calibration may still contain errors. D. Groenveld et al., A Self-calibration Technique for Monolithic High-Resolution D/A Converters, IEEE Journal of Solid-State Circuits, Vol. 24, pp. 1517-1522, December 1989.
It is therefore an object of this invention to provide an improved calibrated current source.
It is a further object of this invention to provide such an improved calibrated current source which insures a more constant current in the load and calibration modes.
It is a further object of this invention to provide such an improved calibrated current source which insures a more constant voltage across the current source in the load and calibration modes.
It is a further object of this invention to provide such an improved calibrated current source which provides both improved isolation and more headroom.
The invention results from the realization that an improved, more accurate calibrated current source can be achieved by employing a cascode switch to switch the current source between the normal load and the calibration circuit so that the voltage across the current source is maintained constant in both the load and calibration modes thereby insuring that the current is the same in both modes and thus the calibrated trimming current added to or subtracted from the current source output is as close as possible to the required correction current making the output more accurate in the load mode.
This invention features a calibrated current source including a current source having an output node, a calibration circuit; and load circuit. There is a cascode switching circuit including a pair of cascode switches one connected between the load circuit and output node, the other connected between the calibration circuit and the output node. A bias circuit selectively applies a bias voltage to the cascode switches to selectively connect the load circuit and the calibration circuit to the output node while maintaining a constant voltage at the output node and across the current source to provide a consistent current to the load and the calibration circuits.
In a preferred embodiment the cascode switches may include FET""s. The current source may include an FET. The load circuit may include an isolation cascode circuit. And the load circuit may include a current switching circuit.